Silicon power semiconductors are manufactured in standardized, yet only in few available voltage ratings, typically 600 V, 1.2 kV, 1.7 kV, 3.3 kV, 4.5 kV and 6.5 kV. The advent of SiC semiconductor devices with blocking voltages in the 10-15 kV range is expected to widen the selection range, nonetheless still being limited. To alleviate this constraint, two main approaches have been followed: series connection of power devices or multi-level converter topologies. For example, a medium voltage power converter can use two series-connected IGBTs (insulated gate bipolar transistors) per functional switch, thus doubling the effective ‘switch’ blocking voltage. This topology requires additional gate-drive and ancillary circuitry, including snubber and active voltage sharing controls as the static and dynamic voltage sharing between devices is of great difficulty and highly critical. The voltage-sharing problem exists regardless of whether semiconductor devices packaged in conventional wire-bond power modules or in press-pack type power modules are used.
The main advantage of press-pack devices, when compared to module-based devices, is that their failure mode is short circuit, which is necessary for the series connection of devices in order to maintain reliability in a series-connected string of devices. In the case of module-based devices, this shortcoming is typically handled by the addition of a bypass thyristor switch across the terminals of a power module, which is standard industrial practice in modular HVDC (high voltage DC) converter stations.
The alternative approach to connecting semiconductors in series is to adopt a multilevel topology. Multilevel topologies can be broadly classified as switching network and modular converter topologies. In the case of switching network topologies, the seemingly series-connected IGBTs only block half of the total DC bus voltage, which is accomplished through the use of clamping power diodes. In the case of modular converter topologies, namely the modular multilevel converter (MMC) and the alternate-arm converter (AAC), the converters themselves are comprised of sub-converters or power converter modules, referred to as power modules or power cells. The power modules float from an electrical standpoint, and thus their voltage rating, including that of their respective semiconductor components, is independent of the total converter rating but instead in terms of ground insulation. Their operation in general terms requires the power modules or power cells to actively participate in the power flow from source to load, acting as transient energy storage devices. As a result, these power modules have high energy storage requirements that increase cost, and also limit the power conversion functionality to AC-AC, AC-DC or DC-AC, thus ruling out their capacity to operate in pure DC-DC mode which significantly limits their capacity to operate in the low frequency range below 10 Hz.